1. Field of Art
This invention relates to apparatus and methods for securing objects to the sea floor. With further particularity, this invention relates to apparatus and methods for embedment type anchors, which present a reduced chance of snagging or hooking upon underwater obstructions, such as pipelines and the like.
2. Description of the Preferred Embodiment
Disclosed are several embodiments of anchors (and improvements thereof), for use in mooring physical objects, including but not limited to floating vessels and structures, and/or grounded structures, typically (but not exclusively) in an marine environment. The anchors, in their various embodiments, have particular (but not exclusive) application in the anchoring of Mobile Offshore Drilling Units (“MODUs”) and related production facilities used in offshore oil and gas operations.
One type of anchor in common use, and well known in the art, is called a High Holding Capacity (“HHC”) anchor, which generally comprises a fluke and a shank. The fluke is generally substantially plate shaped, and in normal conditions, the fluke is substantially horizontal or aligned with the plane of the sea floor, while the shank extends generally upwardly therefrom in a plane generally at right angles to the plane of the fluke. A line, which may be the mooring line to the object being moored, is connected to the shank, and runs to the vessel or structure being moored. After lowering the HHC to the sea floor, tension applied to the line will pull the HHC anchor such that it digs into the sea floor, and eventually buries itself to some design depth. Once the anchor is set, the angle of the mooring line with respect to horizontal is generally relatively small.
Another type of anchor in common use is that known as a Vertically Loaded Anchor (“VLA”). Such anchor typically comprises a plate, the plate being connected to an installation and mooring line. The plate is pulled into the sea floor to a (typically) greater depth than an HHC anchor, and the angle of the mooring line with respect to horizontal is generally greater than the equivalent mooring line angle with an HHC anchor.
There are many pipelines and other obstructions on the sea floor. In the course of installing HHC and VLAs, it can be appreciated that if pulled over a pipeline or other obstruction, the anchor may hook onto or snag same, with potentially disastrous consequences (e.g. ruptured pipelines). Certain weather conditions, such as hurricanes, can impart sufficient forces on the moored vessel or structure so as to overcome the mooring or holding capacity of its anchors, and thereby drag anchors over long distances, again giving rise to the possibility of hooking onto a pipeline or other structure. Even if the anchor is improperly aligned, for example where an HHC anchor is positioned so that the fluke is substantially vertically (as opposed to horizontally) oriented, it is still possible for a fluke “wing” to hook a pipeline or other underwater object.
Another issue exists with conventional HHC anchors. The angle of the shank with respect to the fluke (for example, see FIG. 3, where the referenced angle is annotated as “shank angle”) is fixed. This fixed angular relationship does not allow the anchor to be pulled at optimum angles for all conditions. Ideally, the angle between the shank and the fluke would increase as soil resistance and consequently mooring pull increase, and thereby permit the fluke to assume a steeper dive angle and allow greater resistance to vertical loading in the soil. Current, fixed angle HHC anchors do not permit such behavior.
Still another issue arises when anchors, namely HHC anchors, are retrieved to an MODU and vessels with outboard anchor racks. Conventional HHC anchors are stored by hooking the flukes on a rack; any anchor design which prevents such anchor mounting presents an issue with efficiently and safely carrying the anchors aboard the vessel.